Ink-jet printing systems can generally be divided into those employing a liquid ink, i.e. a water-based or solvent-based ink which is in the liquid state at room temperature, and those employing a so-called hot-melt ink which has its melting point above room temperature, e.g. at 100.degree. C. or more. In both systems, the printhead typically has a flat nozzle face in which a number of orifices are formed through which ink droplets are expelled onto the printing medium either with a constant frequency or "on demand". The printhead further includes pressurizing means for generating the ink droplets by abruptly increasing the pressure of a volume of ink liquid which is contained in a nozzle channel associated with each of the nozzle orifices. In case of a hot-melt system, the printhead additionally includes heating means for keeping the temperature of the ink above its melting point, as long as the printhead is operative.
It is generally known in the art that the directional stability of the ink droplets expelled from the nozzle orifices may be adversely influenced when the surface of the nozzle face around the nozzle orifice is wetted with ink. Such a wetting of the nozzle face also has the drawback that dust particles may be absorbed by the ink at the nozzle face and may be drawn back into the ink channel so that they cause clogging of the nozzle orifice. A well-known remedy to these problems, at least in conjunction with water-based ink systems, is to coat the nozzle face of the printhead with an anti-wetting coating. In relation to a given ink liquid, the coating material should have an adhesive force which is small in comparison to the cohesive force (surface tension) of the liquid. In other words, the wetting angle or angle of contact between the coating and the ink liquid should be comparatively large, preferably on the order of 70.degree. or more. As result, the ink liquid is "repelled" by the coating of the nozzle face, so that an undesired wetting of the nozzle face is avoided.
In conjunction with liquid ink systems, in particular water-based inks, a large number of suitable anti-wetting coating materials are known. For example, U.S. Pat. No. 4,296,421 proposes polyurethane, epoxide resin, phenoxy resin, phenolic resin, and also--among many others--silicone resin as a suitable material for an anti-wetting coating of an ink-jet printhead.
Coating materials which are effective for aqueous inks, however, are not obviously suited for solvent based inks and, in particular, for hot-melt inks. Compared to aqueous inks, hot-melt inks have considerably lower surface tensions. Accordingly, the coating materials should also have a low surface tension, i.e., a large wetting angle. In view of the elevated temperatures under which the hot-melt inks are used, it is further required that the coating materials do not swell or degrade in hot organic solvents. Further, since the hot-melt inks are typically used in printheads in which the ink is pressurized by means of piezoelectric actuators (ceramics), the curing temperature of the coating resin should be low in order to avoid depolarization of the piezoelectric material.
Ordinary organic materials are known to have a poor chemical resistance against hot organic solvents such as hot-melt inks. For example, many silicone-based elastomers and resins will generally swell and may eventually degrade when exposed to such hot-melt inks at elevated temperatures. Among the commonly used coating materials with low surface tension, only fluorinated polymers are typically resistant against organic solvents.
For these reasons, in conjunction with printheads adapted for printing with hot-melt inks, fluorinated inks have been proposed as anti-wetting coatings. For example, EP-A-0 359 365 discloses a printhead that is adapted for printing with hot melt ink in which the nozzle face is provided with an anti-wetting coating of polytetrafluorethylene (PTFE) or similar materials.
However, several drawbacks are associated with the use of fluorinated coating materials. Such fluorinated coatings have to be cured at relatively high temperatures, i.e., 250.degree. C. or more. Such high temperatures may easily lead to depolarization of the piezoelectric actuators and are therefore not allowed in the fabrication process of the printhead.
Furthermore, the adhesion of fluorinated polymers to substrates is generally poor. In order to achieve a sufficient permanency of the coating, an appropriate primer should be applied to the substrate before applying the fluorinated resin. This increases the complexity and the costs of the manufacturing process. Fluorinated polymers further require the use of special solvents during the coating process, which also contributes to increased costs.
Since the scratch resistance of fluorinated polymer coatings is generally known to be poor, there is a high risk that the coating is harmed during procedures for cleaning the printheads.
Although fluorinated coatings initially have a relatively large wetting angle, it has been found that the wetting angle decreases with time, which is probably due to swelling and/or dissolution of the coating. This effect is particularly undesirable from the viewpoint of constant jet behavior of the printhead.